In the conventional processing procedure for a gallium arsenide (GaAs) MESFET shown in FIG. 1, the starting material is a semi-insulating ("SI") single-crystal gallium arsenide substrate 12. An active region n-type layer 14 and an n+ layer 16, are usually molecular beam epitaxially ("MBE") grown or multiple selectively ion implanted. Mesa structures are etched to the SI substrate 12 or selectively proton implanted for isolation of a particular MESFET device.
Nickel/12% germanium-88% gold/nickel/gold, Ni/GeAu/Ni/Au, ohmic contacts 18, 20 are vacuum deposited and alloyed to form a source and a drain. Refractory metal ohmic contacts, e.g., germanium/molybdenum/tungsten, Ge/Mo/W, may also be used for high temperature (i.e., &gt;200.degree. C.) operation. Multiple selective ion implantation may also be used to form such source and drain contacts 18, 20, followed by ohmic metal as described above.
The area between source 18 and drain 20 is usually chemically removed through n+ layer 16 into n-type layer 14, and the Shottky barrier gate metal 22 is vacuum deposited.
A conventional GaAs MESFET deteriorates rapidly above 125.degree. C. as a result of gold diffusion from source and drain ohmic contacts 18, 20 into active region 14. The incorporation of refractory ohmic contacts may be used to eliminate this problem. Operation speed, noise, maximum operating temperature and efficiency of conventional MESFETs are limited due to the structure of a conventional GaAs MESFET. Speed, gain and noise are affected by the distance carriers must travel in the current path 24 between source 18 and drain 20, and by the RC constants inherent in the conventional GaAs MESFET structure. These constraints require the ultimate in design, alignment and dimensional control for fabrication.
There is no ability to tailor the depletion or enhancement mode characteristics of a completed conventional GaAs MESFET. The position of gate 22 in a conventional GaAs MESFET precludes any further modification of current channel 24. The light sensitivity of the conventional MESFET is limited since the light can only enter the device in the constrained areas between gate 22 and source 18, and gate 22 and drain 20, which limits the effectiveness of the conventional GaAs MESFET as a photoreceiver. Moreover, the maximum operating temperature of the device is limited to the temperature sensitivity of the intrinsic carriers in the SI GaAs substrate, which is usually about 125-150.degree. C.
A previous phototransistor invented by Gerald D. Robinson to solve the problem of the temperature sensitivity of the intrinsic carriers in an SI GaAs substrate is illustrated in FIG. 2. In this arrangement the semi-insulating GaAs substrate of a conventional MESFET is removed to alleviate temperature sensitivity and make the phototransistor more robust. The previous phototransistor and a method of making such phototransistor is described in detail in copending U.S. application Ser. No. 08/274,931 entitled "BACKSIDE ILLUMINATED FET OPTICAL RECEIVER AND METHOD WITH GALLIUM ARSENIDE SPECIES" by Gerald D. Robinson, which is incorporated by reference as if fully set forth herein. The previous phototransistor illustrated in FIG. 2 is created from a typical FET structure grown above an AlAs layer above a semi-insulating GaAs substrate. A MESFET is then fabricated using conventional procedures, such as described with reference to FIG. 1. The MESFET is then flipped upside-down and bonded, gate side down, using a thermally-conductive and electrically-insulative high temperature epoxy 32, onto an arbitrary substrate 36 such as aluminum nitride (AlN), aluminum oxide (Al.sub.2 O.sub.3), sapphire, etc. The GaAs semi-insulating substrate 38 is lapped to a thickness of approximately 100 microns. The remaining GaAs is removed, stopping at the AlAs layer 40, using reactive ion etching ("RIE") with Freon 12, CCl.sub.2 F.sub.2, or equivalent, etchant gas. As a result of the flipping, the backside of the fabricated MESFET is exposed. An active layer mesa from the GaAs substrate remains in the center of the exposed backside after the RIE treatment. A Schottky barrier gate 42 consisting of Ti, Pt, and Au, is thus positioned below the thin (approx. 15 micron) active layer 38 of GaAs. The gate 42 and the source and drain ohmic contacts 44, 46 to the n+ GaAs 48 are below the gate.
The previous phototransistor avoids some of the shortcomings of conventional GaAs MESFETs. This previous phototransistor, invented by Gerald D. Robinson, works without degradation to 250.degree. C. and has a high degree of light sensitivity because most of the thin exposed backside over the gate is removed. Removal of semi-insulating substrate 38 increases the maximum operating temperature of the previous phototransistor to greater than 300.degree. C., and also makes the previous phototransistor an effective photoreceiver because light can impinge directly on the thin exposed backside above gate 42.
However, some deficiencies associated with conventional MESFETs exist in the previous phototransistor. The transit time for carriers to flow in the current path 49 from the source to the drain is still relatively long. The RC constants inherent in the conventional GaAs MESFET have not been eliminated by the design of the previous phototransistor. The previous phototransistor is bandwidth limited to the photosensitivity of the GaAs layer, which is in a range from 0.6-0.9 mm wavelength. The speed, noise and gain of conventional MESFETs (FIG. 1) and the previous phototransistor (FIG. 2) are primarily limited by the carrier transit time in the current path from the source to the drain and the inherent RC constants of the devices.